Magnetic powder and isotropic bonded magnet

ABSTRACT

Disclosed herein is magnetic powder which can provide a magnet having a high magnetic flux density and excellent magnetizability and reliability especially excellent heat resistance property (heat stability). The magnetic powder is composed of an alloy composition represented by R x (Fe 1-y Co y ) 100-x-z-w B x Si w  (where R is at least one kind of rare-earth element, x is 8.1-9.4 at %, y is 0-0.30, z is 4.6-6.8 at %, and w is 0.2-3.0 at %), the magnetic powder being constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, wherein the magnetic powder has characteristics in which, when the magnetic powder is formed into an isotropic bonded magnet by mixing with a binding resin and then molding it, the irreversible susceptibility (χ irr ) which is measured by using an intersection of a demagnetization curve in the J-H diagram representing the magnetic characteristics at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of −3.8×10 −6  H/m as a starting point is less than 5.0×10 −7  H/m, and the intrinsic coercive force (H CJ ) of the magnet at the room temperature is in the range of 406-717 kA/m.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetic powder and an isotropic bondedmagnet produced using the magnetic powder.

2. Description of the Prior Art

For reduction in size of motors, it is desirable that a magnet has ahigh magnetic flux density (with the actual permeance) when it is usedin the motor. Factors for determining the magnetic flux density of abonded magnet include magnetic performance (that is, magnetization) ofthe magnetic powder and the content (that is, compositional ratio) ofthe magnetic powder in the bonded magnet. Accordingly, when the magneticperformance (magnetization) of the magnetic powder itself is notsufficiently high, a desired magnetic flux density cannot be obtainedunless the content of the magnetic powder in the bonded magnet is raisedto an extremely high level.

At present, most of practically used high performance rare-earth bondedmagnets use the isotropic bonded magnets which are made using MQP-Bpowder manufactured by MQI Corp. as the rare-earth magnetic powderthereof. The isotropic bonded magnets are superior to the anisotropicbonded magnets in the following respect; namely, in the manufacture ofthe bonded magnet, the manufacturing process can be simplified becauseno magnetic field orientation is required, and as a result, the rise inthe manufacturing cost can be restrained. On the other hand, however,the conventional isotropic bonded magnets such as those manufacturedusing MQP-B powder having the following disadvantages.

(1) The conventional isotropic bonded magnets do not have sufficientlyhigh magnetic flux density. Specifically, because of the low magneticperformance (that is, the insufficient magnetization) of the magneticpowder used, the content of the magnetic powder to be contained in thebonded magnet has to be increased. However, the increase in the contentof the magnetic powder leads to the deterioration in the moldability ofthe bonded magnet, so there is a certain limit in this attempt.Moreover, even if the content of the magnetic powder is somehow managedto be increased by changing the molding conditions or the like, therestill exists a limit to the obtainable magnetic flux density. For thesereasons, it is not possible to reduce the size of the motor by using theconventional isotropic bonded magnets.

(2) Since the conventional bonded magnet has high coercivity (coerciveforce), magnetizability thereof is poor, thus requiring a relativelyhigh magnetic field of magnetization.

(3) Although there are reports concerning nanocomposite magnets havinghigh remanent magnetic flux densities, their coercive forces, on thecontrary, are so small that the magnetic flux densities (for thepermeance in the actual use) obtainable for the practical motors arevery low. Further, these magnets have poor heat stability due to theirsmall coercive forces.

(4) The conventional bonded magnets have low temperature characteristics(that, is heat resisting property or heat stability). In particular, theirreversible flux loss remarkably drops when the coercive force (H_(CJ))is lowered.

(5) The bonded magnet has poor corrosion resistance and heat resistingproperty. In particular, when the content of the magnetic is increasedin order to compensate the low magnetic performance of the magneticpowder (that is, when the magnetic flux density of the bonded magnet isextremely raised), the corrosion resistance and heat resisting propertyremarkably drop.

Therefore, it is necessary to cover the outer surface of the bondedmagnet with a coating, especially a resin coating which is capable ofobtaining high corrosion resistance property, but this in turn leads toincreased manufacturing cost and the presence of the resin layer resultsin lowered magnetic performance (this makes it difficult for a motor togenerate a high torque).

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide magneticpowder that can produce a magnet having a high magnetic flux density andhaving excellent magnetizability and reliability especially corrosionresistance property and temperature characteristics (that is, heatresisting property and heat stability), and provide an isotropic bondedmagnet formed from the magnetic powder.

In order to achieve the above object, the present invention is directedto magnetic powder having an alloy composition represented byR_(x)(Fe_(1-y)Co_(y))_(100-x-x-w)B_(X)Si_(w) (where R is at least onekind of rare-earth element, x is 8.1-9.4 at %, y is 0-0.30, z is 4.6-6.8at %, and w is 0.2-3.0 at %), the magnetic powder being constituted froma composite structure having a soft magnetic phase and a hard magneticphase, wherein the magnetic powder has characteristics in which, whenthe magnetic powder is formed into an isotropic bonded magnet by mixingwith a binding resin and then molding it, the irreversiblesusceptibility (χ_(irr)) which is measured by using an intersectioningpoint of a demagnetization curve in the J-H diagram representing themagnetic characteristics at the room temperature and a straight linewhich passes the origin in the J-H diagram and has a gradient (J/H) of−3.8×10⁻⁶ H/m as a starting point is less than 5.0×10⁻⁷ H/m, and theintrinsic coercive force (H_(CJ)) of the magnet at the room temperatureis in the range of 406-717 kA/m.

As described above, according to the present invention, the followingeffects can be obtained.

Since each of the magnetic powders has the composite structure having asoft magnetic phase and a hard magnetic phase and contains apredetermined amount of Si, the magnetic powder exhibits excellentmagnetic characteristics so that intrinsic coercive force andrectangularity thereof are especially improved, and excellent corrosionresistance property is exhibited.

The absolute value of the irreversible flux loss is small and excellentheat resisting property (heat stability) can be obtained.

Because of the high magnetic flux density that can be secured by thisinvention, it is possible to obtain a bonded magnet with high magneticperformance even if it is isotropic. In particular, since magneticperformance equivalent to or better than the conventional isotropicbonded magnet can be obtained with a magnet of smaller volume ascompared with the conventional isotropic bonded magnet, it is possibleto provide a high performance motor of a smaller size.

Moreover, since a higher magnetic flux density can be secured, inmanufacturing a bonded magnet sufficiently high magnetic performance isobtainable without pursuing any means for elevating the density of thebonded magnet. As a result, the dimensional accuracy, mechanicalstrength, corrosion resistance, heat resisting property (heat stability)and the like can be further improved in addition to the moldability, sothat it is possible to readily manufacture a bonded magnet with highreliability.

Since the magnetizability of the magnet according to this invention isexcellent, it is possible of magnetize a magnet with a lower magnetizingfield. In particular, multipolar magnetization or the like can beaccomplished easily and surely, and further a high magnetic flux densitycan be obtained.

Since the bonded magnet of the present invention does not require tohave a high density, the present invention is adapted to themanufacturing method such as the extrusion molding method or theinjection molding method by which molding at high density is difficultas compared with the compression molding method, and the effectsdescribed in the above can also be obtained in the bonded magnetmanufactured by these molding methods. Accordingly, various moldingmethods can be selectively used and thereby the degree of selection ofshapes for the bonded magnet can be expanded.

In this connection, it is preferred that the composite structure is ananocomposite structure.

Further, in the present invention, it is preferred that said R comprisesrare-earth elements mainly containing Nd and/or Pr. In this case, it ispreferable that said R includes Pr and its ratio with respect to thetotal mass of said R is 5-75%. When the ratio lies within this range, itis possible to improve the coercivity and the rectangularity by hardlycausing a drop in the remanent magnetic flux density.

Furthermore, in the present invention, it is also preferred that said Rincludes Dy and its ratio with respect to the total mass of said R isequal to or less than 14%. When the ratio lies within this range, thecoercivity can be improved without causing marked drop in the remanentmagnetic flux density, and the heat resisting property is also improved.

Moreover, it is preferred that the magnetic powder is obtained byquenching the ally of a molten state.

It is also preferred that the magnetic powder is obtained by pulverizinga quenched ribbon of the alloy which is manufactured by using a coolingroll.

Further, it is also preferred that the magnetic powder is subjected to aheat treatment for at least once during the manufacturing process orafter its manufacture.

Furthermore, it is also preferred that the average grain size of themagnetic powder lies in the range of 0.5-150 μm.

The present invention is also directed to an isotropic rare-earth bondedmagnet, which is formed by binding the magnetic powder as set forth inthe above with a binding resin.

The present invention is also directed to an isotropic rare-earth bondedmagnet formed by binding magnetic powder with a binding resin, whereinthe isotropic rare-earth bonded magnet is characterized in that theirreversible susceptibility (χ_(irr)) which is measured by using anintersectioning point of a demagnetization curve in the J-H diagramrepresenting the magnetic characteristics at the room temperature and astraight line which passes the origin in the J-H diagram and has agradient (J/H) of −3.8×10⁻⁶ H/m as a starting point is equal to or lessthan 5.0×10⁻⁷ H/m, and the intrinsic coercive force (H_(CJ)) of themagnet at the room temperature is in the range of 406-717 kA/m.

In this case, it is preferred that the magnetic powder used in theisotropic bonded magnet includes Si.

Further, it is preferred that said magnetic powder is formed ofR-TM-B-Si based alloy (where R is at least one rear-earth element and TMis a transit metal containing Iron as a major component thereof).

Moreover, it is preferred that the magnetic powder has a compositestructure having a soft magnetic phase and a hard magnetic phase.

Moreover, it is also preferred that the isotropic bonded magnet is to besubjected to multipolar magnetization or is already subjected tomultipolar magnetization. In this case, it is also preferred that theisotropic bonded magnet is used for a motor.

These and other objects, structures and advantages of the presentinvention will be apparent from the following detailed description ofthe invention and the examples taken in conjunction with the appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration which schematically shows one example of acomposite structure (nanocomposite structure) of a magnetic powderaccording to the present invention.

FIG. 2 is an illustration which schematically shows one example of acomposite structure (nanocomposite structure) of a magnetic powderaccording to the present invention.

FIG. 3 is an illustration which schematically shows one example of acomposite structure (nanocomposite structure) of a magnetic powderaccording to the present invention.

FIG. 4 is a perspective view showing an example of the configuration ofan apparatus (quenched ribbon manufacturing apparatus) for manufacturinga magnet material.

FIG. 5 is a sectional side view showing the situation in the vicinity ofcolliding section of the molten metal with a cooling roll in theapparatus shown in FIG. 4.

FIG. 6 is a J-H diagram (coordinate) for explaining the irreversiblesusceptibility.

FIG. 7 is a J-H diagram (coordinate) that represents demagnetizationcurves and recoil curves.

FIG. 8 is a table showing the results of a first example of the presentinvention.

FIG. 9 is a table showing the results of a second example of the presentinvention.

FIG. 10 is a table showing the results of a third example of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, magnetic powder according to this invention and anisotropic rare-earth bonded magnet using the magnetic powder will bedescribed in detail.

At present, a magnet having a high magnetic flux density is practicallyrequired in order to reduce the size of motors or other electricaldevices. In a bonded magnet, factors that determine the magnetic fluxdensity are the magnetic performance (magnetization, in particular) ofmagnetic powder used and the content (compositional ratio) of themagnetic powder contained in the bonded magnet. When the magneticperformance (magnetization) of the magnetic powder itself is not sohigh, a desired magnetic flux density cannot be obtained unless thecontent of the magnetic powder contained in the bonded magnet isincreased to an extremely high level.

As mentioned in the above, MQP-B powder made by MQI Corp. which is nowbeing widely used can not provide sufficient magnetic flux density. As aresult, in manufacturing the bonded magnets, it is required to increasethe content of the magnetic powder contained in the bonded magnet, thatis, it is required to increase the magnetic flux density. However, thisin turn leads to the lack of reliability in the corrosion resistanceproperty and mechanical strength thereof and the like. Further, there isa problem in that the obtained magnet has a poor magnetizability due toits high coercivity.

In contrast, the magnetic powder and the isotropic bonded magnet(isotropic rear-earth bonded magnet) according to this invention canobtain a sufficient magnetic flux density and an adequate coerciveforce. As a result, without extremely increasing the content of themagnetic powder contained in the bonded magnet, it is possible toprovide a bonded magnet having high strength and having excellentmoldability, corrosion resisting property, durability andmagnetizability. This makes it possible to reduce the size of the bondedmagnet and increase its performance, thereby contributing to reduce thesize of motors and other electrical devices employing magnets.

Further, the magnetic powder of the present invention may be formed soas to constitute a composite structure having a hard magnetic phase anda soft magnetic phase.

While the MQP-B powder manufactured by MQI Corp. is a single phasestructure of a hard magnetic phase, the magnetic powder of the presentinvention is a nanocomposite structure which also has a soft magneticphase having high magnetization. Accordingly, the bonded magnet of thepresent invention has an advantage that the total magnetization of thesystem as a whole is high. Further, since the recoil permeability of thebonded magnet becomes high, there is an advantage that, even after areverse magnetic field is applied, the demagnetizing factor remainssmall.

The magnetic powder according to this invention has alloy compositionsrepresented by R_(x)(Fe_(1-y)Co_(y))_(100-x-x-w)B_(x)Si_(w) (R is atleast one kind of rare-earth element, x is 8.1-9.4 at %, y is 0-0.30, xis 4.6-6.8 at %, and w is 0.2-3.0 at %).

Examples of rare-earth metals R include Y, La, Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and a misch metal. In this connection, Rmay include one kind or two or more kinds of these elements.

The content of R is set at 8.1-9.4 at %. When the content of R is lessthan 8.1 at %, sufficient coercive force cannot be obtained, andaddition of Si enhances the coercive force only to a small extent. Onthe other hand, when the content of R exceeds 9.4 at %, sufficientmagnetic flux density fails to be obtained because of the drop in themagnetization potential.

Here, it is preferable that R includes the rare-earth elements Nd and/orPr as the principal ingredient. The reason for this is that theserare-earth elements enhance the saturation magnetization of the hardmagnetic phase which constitutes the composite structure (especially,nanocomposite structure) the magnetic powder, and are effective inrealizing satisfactory coercive force as a magnet.

Moreover, it is preferable that R includes Pr, and its ratio to thetotality of R is 5-75%, and more preferably 10-60%. This is because whenthe ratio lies in this range, it is possible to improve the coercivityand the rectangularity by hardly causing a drop in the remanent magneticflux density.

Furthermore, it is preferable that R includes Dy and its ratio to thetotality of R is equal to or less than 14%. When the ratio lies in thisrange, the coercivity can be improved without causing marked drop in theremanent magnetic flux density, and an improvement of the heat resistingproperty is also possible.

Cobalt (Co) is a transition metal element having properties similar toFe. By adding Co, that is by substituting a part of Fe by Co, the Curietemperature is elevated and the temperature characteristic of the powderis improved. However, if the substitution ratio of Fe by Co exceeds0.30, both of the coercive force and the magnetic flux density tend tofall off. The range of 0.05-0.20 of the substitution ratio of Fe by Cois more preferable since in this range not only the temperaturecharacteristic of the magnetic powder but also the magnetic flux densitythereof are improved. In this regard, it is to be noted that a part ofFe or Co may be substituted by Ni.

Boron (B) is an element which is important for obtaining high magneticcharacteristics, and its content is set at 4.6-6.8 at %. When thecontent of B is less than 4.6 at %, the rectangularity of the magneticpower in the J-H diagram is deteriorated. On the other hand, when thecontent of B exceeds 6.8 at %, the nonmagnetic phase increase and themagnetic flux density drops sharply.

Silicon (Si) is an element which is advantageous in improving corrosionresistance property of the magnetic powder and the bonded magnet, andsuch effect can be seen by adding Si in an amount of 0.2 to 3.0 at %.Further, by adding Si of this range, the coercive force is improved, andin accompany with this effect, the rectangularity and the maximummagnetic energy product are also improved. Another important effectwhich is achieved by adding 0.2 to 3.0 at % of Si is that theirreversible flux loss can be improved. In this connection, if theamount of Si is less than 0.2 at %, the effect that improves corrosionresistance property can be hardly seen. On the other hand, if the amountof Si exceeds 3.0 at %, the magnetizability is markedly lowered andtherefore it is not preferable. In this connection, more preferablerange of the content of Si is 0.5 to 2.0 at %.

Of course, Si itself is a known substance. However, in the presentinvention, it has found through repeatedly conducted experiments andresearches that by containing 0.2-3.0 at % of Si to the magnetic powderconstituted from a composite structure having a soft magnetic phase anda hard magnetic phase, the following three effects are realized, inparticular these three effects are realized at the same time, and thisis the significance of the present invention.

(1) The corrosion resistance property can be improved.

(2) The coercive force of the magnetic powder can be improved whilemaintaining excellent rectangularity and the maximum magnetic energyproduct.

(3) The irreversible flux loss can be improved, that is the absolutevalue thereof can be lowered.

As described in the above, the feature of this invention can be found inthe addition of a minute amount or trace amount of Si, and the additionof Si in the amount exceeding 3.0 at % gives rather an inverse effectand therefore it is not the intent of this invention.

In addition, for the purpose of further improving the magneticcharacteristics, at least one other element selected from the groupcomprising Cu, Al, Ga, Ti, V, Ta, Zr, Nb, Mo, Hf, Ag, Zn, P and Ge andthe like may be obtained in the alloy which forms the magnetic powder.

As described above, the magnetic material of the present invention has acomposite structure having a soft magnetic phase and a hard magneticphase.

In this composite structure (nanocomposite structure), a soft magneticphase 10 and a hard magnetic phase 11 exist in a pattern (model) asshown in, for example, FIG. 1, FIG. 2 or FIG. 3, where the thickness orgrain diameter of the respective phases is on the order of nanometers(for example, 1-100 nm). Further, the soft magnetic phase 10 and thehard magnetic phase 11 are arranged adjacent to each other (this alsoincludes the case where these phases are adjacent through grain boundaryphase), which make it possible to perform magnetic exchange interactiontherebetween.

In this case, the average grain diameter size is preferably 5 to 50 nmand more preferably 10 to 40 nm. In the case where the average crystalgrain size (diameter) is less than the lower limit value, the influenceof the magnetic exchange interaction becomes too strong and therebymagnetic inversion is likely to occur, which may result in the casewhere the coercive force is deteriorated. On the other hand, when theaverage crystalline grain size exceeds the upper limit value, thecrystalline grain size becomes large while the influence of the magneticexchange interaction between crystalline grains is weakened, thusleading to the case that the magnetic flux density, coercive force andrectangularity and maximum energy product may deteriorated.

The patterns illustrated in FIG. 1 to FIG. 3 are only specific examples,and are not limited thereto. For example, the soft magnetic phase 10 andthe hard magnetic phase 11 in FIG. 2 are interchanged.

The magnetization of the soft magnetic phase readily changes itsorientation by the action of an external magnetic field. Therefore, whenit coexists with the hard magnetic phase, the magnetization curve forthe entire system shows a stepped “serpentine curve” in the secondquadrant of the J-H diagram. However, when the soft magnetic phase has asufficiently small size of less than several tens of nm, magnetizationof the soft magnetic body is sufficiently strongly constrained throughthe coupling with the magnetization of the surrounding hard magneticbody, so that the entire system exhibits functions like a hard magneticbody.

A magnet having such a composite structure (nanocomposite structure) hasmainly the following five features.

(1) In the second quadrant of the J-H diagram (that is, coordinate wherethe longitudinal axis represents magnetization (J) and the horizontalaxis represents magnetic field (H)), the magnetization springs backreversively (in this sense, such a magnet is also referred to as a“spring magnet”).

(2) It has a satisfactory magnetizability, and it can be magnetized witha relatively low magnetic field.

(3) The temperature dependence of the magnetic characteristics are smallas compared with the case where the system is constituted from a hardmagnetic phase alone.

(4) The changes in the magnetic characteristics with the lapse of timeare small.

(5) No deterioration in the magnetic characteristics is observable evenif it is finely pulverized.

In the alloy composition described in the above, the hard magnetic phaseand the soft magnetic phase are composed of, for example, respectivelyby the following.

The hard magnetic phase: R₂TM₁₄B system (where, TM is Fe or Fe and Co),or R₂TM₁₄BSi system.

The soft magnetic phase: TM (α-Fe or α-(Fe, Co) in particular), or analloy of TM and Si.

As for the magnetic powder according to this invention, it is preferablethat they are manufactured by quenching a molten alloy, and morepreferable that they are manufactured by pulverizing a quenched ribbonobtained by quenching and solidifying the molten metal of the alloy. Anexample of such a method will be described in the following.

FIG. 4 is a perspective view showing an example of the configuration ofan apparatus (quenched ribbon manufacturing apparatus) for manufacturinga magnet material by the quenching method using a single roll, and FIG.5 is a sectional side view showing the situation in the vicinity ofcolliding section of the molten metal with a cooling roll in theapparatus shown in FIG. 4.

As shown in FIG. 4, a quenched ribbon manufacturing apparatus 1 isprovided with a cylindrical body 2 capable of storing the magnetmaterial, and a cooling roll 5 which rotates in the direction of anarrow 9A in the figure relative to the cylindrical body 2. A nozzle(orifice) 3 which injects the molten metal of the magnet material alloyis formed at the lower end of the cylindrical body 2.

In addition, a heating coil 4 is arranged on the outer periphery of thecylindrical body 2 in the vicinity of the nozzle 3, and the magnetmaterial in the cylindrical body 2 is melted by inductively heating theinterior of the cylindrical body 2 through application of, for example,a high frequency wave to the coil 4.

The cooling roll 5 is constructed from a base part 51 and a surfacelayer 52 which forms a circumferential surface 53 of the cooling roll 5.

The base part 51 may be formed either integrally with the surface layer52 using the same material, or formed using a material different fromthat of the surface layer 52.

Although there is no particular limitation on the material of the basepart 51, it is preferable that it is a metallic material with high heatconductivity such as copper or a copper alloy in order to be able todissipate heat of the surface layer 52 as quickly as possible.

Further, it is preferable that the surface layer 52 is constituted of amaterial with heat conductivity which is slightly lower than that of thebase part 51.

The quenched ribbon manufacturing apparatus 1 is installed in a chamber(not shown), and is operated preferably under the condition where theinterior of the chamber is filled with an inert or other kind of gas. Inparticular, in order to prevent oxidation of a quenched ribbon 8, it ispreferable that the gas is an inert gas such as argon, helium, nitrogenor the like.

In the quenched ribbon manufacturing apparatus 1, the magnet material(alloy) is placed in the cylindrical body 2 and melted by heating withthe coil 4, and the molten metal 6 is discharged from the nozzle 3.Then, as shown in FIG. 5, the molten metal 6 collides with thecircumferential surface 53 of the cooling roll 5, and after theformation of a puddle 7, the molten metal 6 is cooled down rapidly to besolidified while dragged along the circumferential surface 53 of therotating cooling roll 5, thereby forming the quenched ribbon 8continuously or intermittently. Roll surface 81 of the quenched ribbon 8thus formed is soon released from the circumferential surface 53, andproceeds in the direction of an arrow 9B in FIG. 4. The solidificationinterface 71 of the molten metal is indicated by a broken line in FIG.5.

The optimum range of the circumferential velocity of the cooling roll 5depends upon the composition of the molten alloy, the wettability of thecircumferential surface 53 with respect to the molten metal 6, and thelike. However, for the enhancement of the magnetic characteristics, avelocity in the range of 1 to 60 m/s is normally preferable, and 5 to 40m/s is more preferable. If the circumferential velocity of the coolingroll 5 is too small, the thickness t of the quenched ribbon 8 is toolarge depending upon the volume flow rate (volume of the molten metaldischarged per unit time), and the diameter of the crystalline grainstends to increase. On the contrary, if the circumferential velocity istoo large, amorphous structure becomes dominant. Further, enhancement ofthe magnetic characteristics can be expected in neither case even if aheat treatment is given in the later stage.

Thus obtained quenched ribbon 8 may be subjected to a heat treatment forthe purpose of, for example, acceleration of recrystallization of theamorphous structure and homogenization of the structure. The conditionsof this heat treatment may be, for example, a heating in the range of400 to 900° C. for 0.5 to 300 min.

Moreover, in order to prevent oxidation, this heat treatment ispreferable to be performed in a vacuum or under a reduced pressure (forexample, in the range of 1×10⁻¹ to 1×10⁻⁶ Torr), or in a nonoxidizingatmosphere of an inert gas such as nitrogen, argon, helium or the like.

The quenched ribbon (thin ribbon-like magnet material) 8 obtained as inthe above has a microcrystalline structure or a structure in whichmicrocrystals are included in an amorphous structure, and exhibitsexcellent magnetic characteristics. The magnetic powder of thisinvention is obtained by pulverizing the quenched ribbon 8.

The pulverizing method of the quenched ribbon is not particularlylimited, and various kinds of pulverizing or crushing apparatus such asball mill, vibration mill, jet mill, and pin mill may be employed. Inthis case, in order to prevent oxidation, pulverization may be performedunder vacuum or reduced pressure (for example, under a low pressure of1×10³¹ ¹ to 1×10⁻⁶ Torr), or in a nonoxidizing atmosphere of an inertgas such as nitrogen, argon, helium, or the like.

The average grain size of the magnetic powder is not particularlylimited. However, for magnetic powder to be used for manufacturingisotropic bonded magnets described later, by considering prevention ofoxidation of the magnetic powder and of deterioration in the magneticcharacteristics due to pulverization, it is preferable to choose therange of 0.5 to 150 μm, more preferably the range of 0.5 to 100 μm, andstill more preferably the range of 1.0 to 65 μm, and most preferably therange of 5.0 to 55 μm.

In order to obtain a better moldability of the bonded magnet, it ispreferable to give a certain degree of dispersion to the grain sizedistribution of the magnetic powder. By so doing, it is possible toreduce the porosity of the bonded magnet obtained. As a result, it ispossible to raise the density and the mechanical strength of the bondedmagnet assuming the same content of the magnetic powder in the bondedmagnet, thereby enabling to further improve the magneticcharacteristics.

The obtained magnetic powder may be subjected to a heat treatment forthe purpose of, for example, removing the influence of stress introducedby the pulverization and controlling the crystalline grain size. Theconditions of the heat treatment are, for example, heating at atemperature in the range of 350 to 850° C. for 0.5 to 300 min.

In order to prevent oxidation of the powder, it is preferable to performthe heat treatment in a vacuum or under a reduced pressure (for example,in the range of 1×10⁻¹ to 1×10⁻⁶ Torr), or in a nonoxidizing atmosphereof an inert gas such as nitrogen, argon, and helium.

When a bonded magnet is manufactured using the magnetic powder describedin the above, the obtained magnetic powder has a satisfactorybindability with the binding resin (wettability of the binding resin),so that the bonded magnet has a high mechanical strength and excellentthermal stability (heat resistance) and corrosion resistance.Consequently, it can be concluded that the magnetic powder is suitablefor the manufacture of the bonded magnet.

In the above, the quenching method is described in terms of the singleroll method, but the twin roll method may also be employed. Besides,other methods such as the atomizing method which uses gas atomization,the rotating disk method, the melt extraction method, and the mechanicalalloying method (MA) may also be employed. Since such a quenching methodcan refine the metallic structure (crystalline grains), it is effectivefor enhancing the magnet characteristics, especially the coercivity orthe like, of the bonded magnet.

Next, the isotropic rare-earth bonded magnets (hereinafter, referred tosimply also as “bonded magnets”) according to this invention will bedescribed.

The bonded magnets of this invention is formed by binding the abovedescribed magnetic powder using a binding resin.

As for the binder, either of a thermoplastic resin or a thermosettingresin may be employed.

As the thermoplastic resin, for example, a polyamid (example: nylon 6,nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12, nylon6-12, nylon 6-66, nylon 6T and nylon 9T); a thermoplastic polyimide; aliquid crystal polymer such as an aromatic polyester; a poly phenyleneoxide; a poly phenylene sulfide; a polyolefin such as a polyethylene, apolypropylene and an ethylene-vinyl acetate copolymer; a modifiedpolyolefin; a polycarbonate; a poly methyl methacrylate; a polyestersuch as a poly ethylen terephthalate and a poly butylene terephthalate;a polyether; a polyether ether ketone; a polyetherimide; a polyacetal,or the like; and a copolymer, a blended body, and a polymer alloy havingthese as main ingredients, or the like, may be mentioned, where one kindor a mixture of two or more kinds of these may be employed.

Among these resins, a resin containing a polyamide as its mainingredient is particularly preferred from the viewpoint of especiallyexcellent moldability and high mechanical strength. Further, a resincontaining a liquid crystal polymer and/or a poly phenylene sulfide asits main ingredient is also preferred from the viewpoint of enhancingthe heat resistance. These thermoplastic resins also have an excellentkneadability with the magnetic powder.

These thermoplastic resins provide an advantage in that a wide range ofselection can be made. For example, it is possible to provide athermoplastic resin having a good moldability or to provide athermoplastic resin having good heat resistance and mechanical strengthby appropriately selecting their kinds, copolymerization or the like.

On the other hand, as the thermosetting resin, various kinds of epoxyresins of bisphenol type, novolak type, and naphthalene-based, aphenolic resin, a urea resin, a melamine resin, a polyester (or anunsaturated polyester) resin, a polyimide resin, a silicone resin, apolyurethane resin, or the like, for example, may be mentioned, and onekind or a mixture of two or more kinds of these may be employed.

Among these resins, an epoxy resin, a phenolic resin, a polyimide resin,or a silicone resin is preferable from the viewpoint of their specialexcellence in the moldability, high mechanical strength, and high heatresistance, and an epoxy resin is especially preferable. Thesethermosetting resins also have an excellent kneadability with themagnetic powder and homogeneity in kneading.

The unhardened thermosetting resin to be used may be either in liquidstate or in solid (powdery) state at room temperature.

A bonded magnet according to this invention described in the above maybe manufactured, for example, as in the following. First, a bondedmagnet composite (compound) which contains the magnetic powder, a binderresin, and an additive (antioxidant, lubricant, or the like) as needed,is prepared. Then, the prepared compound is formed into a desired magnetform in a space free from magnetic field by a molding method such ascompression molding (press molding), extrusion molding, or injectionmolding. When the binding resin used is a thermosetting type, theobtained green compact is hardened by heating or the like after molding.

In the three kinds of molding method, the extrusion molding and theinjection molding (in particular, the injection molding) have advantagesin that the latitude of shape selection is broad, the productivity ishigh, and the like. However, these molding methods require to ensure asufficiently high fluidity of the compound in the molding machine inorder to obtain satisfactory moldability. For this reason, in thesemethods it is not possible to increase the content of the magneticpowder, namely, to make the bonded magnet having high density, ascompared with the case of the compression molding method. In thisinvention, however, it is possible to obtain a high magnetic fluxdensity as will be described later, so that excellent magneticcharacteristics can be obtained even without making the bonded magnethigh density. This advantage of the present invention can also beextended even in the case where bonded magnets are manufactured by theextrusion molding method or the injection molding method.

The content of the magnetic powder in the bonded magnet is notparticularly limited, and it is normally determined by considering thecompatibility of the molding method and moldability, and high magneticcharacteristics. More specifically, it is preferable to be in the rangeof 75-99 wt %, and more preferably in the range of 85-97.5 wt %.

In particular, for a bonded magnet to be manufactured by the compressionmolding method, the content of the magnetic powder is preferable to bein the range of 90-99 wt %, and more preferably in the range of 93-98.5wt %.

For a bonded magnet to be manufactured by the extrusion molding or theinjection molding, the content of the magnetic powder is preferable tobe in the range of 75-98 wt %, and more preferably in the range of 85-97wt %.

The density ρ of the bonded magnet is determined by factors such as thespecific gravity of the magnetic powder contained in the magnet and thecontent of the magnetic powder, and porosity of the bonded magnet andthe like. In the bonded magnets according to this invention, the densityρ is not particularly limited, but it is preferable to be in the rangeof 5.3-6.6 g/cm³, and more preferably in the range of 5.5-6.4 g/cm³.

In this invention, since the magnetic flux density and the coerciveforce of the magnetic powder are high and the magnetic powder has arelatively larger coercive force, the molded bonded magnet providesexcellent magnetic characteristics (especially, high maximum magneticenergy product) even when the content of the magnetic powder isrelatively low. In this regard, it goes without saying that it ispossible to obtain the excellent magnetic characteristics in the casewhere the content of the magnetic powder is high.

The shape, dimensions, and the like of the bonded magnet manufacturedaccording to this invention are not particularly limited. For example,as to the shape, all shapes such as columnar, prism-like, cylindrical(ring-shaped), circular, plate-like, curved plate-like, and the like areacceptable. As to the dimensions, all sizes starting from large-sizedone to ultraminuaturized one are acceptable. However, as repeatedlydescribed in this specification, the present invention is particularlyadvantageous in miniaturization and ultraminiaturization of the bondedmagnet.

The bonded magnet of this invention as described in the above hasmagnetic characteristics in which the irreversible susceptibility(χ_(irr)) which is measured by using an intersection of ademagnetization curve in the J-H diagram (that is, coordinate where thelongitudinal axis represents magnetization (J) and the horizontal axisrepresents magnetic field (H)) representing the magnetic characteristicsat the room temperature and a straight line which passes the origin inthe J-H diagram and has a gradient (J/H) of −3.8×10⁻⁶ H/m as a startingpoint is less than 5.0×10 ⁻⁷ H/m, and the intrinsic coercive force(H_(CJ)) of the magnet at the room temperature is in the range of406-717 kA/m. Hereinafter, explanation will be made with regard toirreversible susceptibility (χ_(irr)) and the intrinsic coercive force(H_(CJ)).

As shown in FIG. 6, the irreversible susceptibility (χ_(irr)) is theparameter which is represented by the following formula (unit isHenry/m, which is represented by H/min in this specification), wherein agradient of a tangential line of the demagnetization curve at a certainpoint P on the demagnetization curve in the J-H diagram is defined bydifferential susceptibility (χ_(dif)) and a gradient of a recoil curvewhen the recoil curve from the point P is drawn with the demagnetizationfield being once reduced (that is, a gradient connecting the both endsof the recoil curve) is defined by reversible susceptibility (χ_(rev)).

Irreversible Susceptibility (χ_(irr))=differential susceptibility(χ_(dif))−reversible susceptibility(χ_(rev))

In this connection, it is to be noted that in the present invention, thepoint P is defined as an intersection of the demagnetization curve and astraight line y which passes the origin in the J-H diagram and has agradient (J/H) of −3.8×10⁻⁶ H/m.

The reason why the upper limit value of the irreversible susceptibility(χ_(irr)) at the room temperature is defined as 5.0×10⁻⁷ H/m is asfollows.

As stated in the above, the irreversible susceptibility (χ_(irr))represents the changing ratio of the magnetization with respect to themagnetic field, which is not returned even if the absolute value thereofis reduced once after demagnetization is applied. Accordingly, byrestraining the irreversible susceptibility (χ_(irr)) to a relativelysmall value, it is possible to improve heat stability of the bondedmagnet and especially to reduce the absolute value of the irreversibleflux loss. Actually, within this range of the irreversiblesusceptibility (χ_(irr)) of the present invention, the irreversible fluxloss obtained when the bonded magnet is being left in the atmosphere of100° C. for one hour and then the temperature is lowered into roomtemperature is equal to or less than 5% in its absolute value, whichmeans that practically satisfactory heat resisting property (inparticular when used in motors or the like), that is heat stability canbe obtained.

In contrast, when the irreversible susceptibility (χ_(irr)) exceeds5.0×10⁻⁷ H/m, the absolute value of the irreversible flux loss isincreased, so that it is not possible to obtain satisfactory heatstability. Further, since the intrinsic coercive force (H_(CJ)) islowered and the rectangularity thereof becomes poor, use of the obtainedbonded magnet is limited to the case where the permeance coefficient(Pc) becomes large (e.g. Pc≧5). Furthermore, the lowered coercive forcereduces the heat stability.

The reason why the irreversible susceptibility (χ_(irr)) at the roomtemperature is defined as 5.0×10⁻⁷ H/m is described above. However, itis preferred that the value of the irreversible susceptibility (χ_(irr))is as smaller as possible. Therefore, in the present invention, it ispreferable that the irreversible susceptibility (χ_(irr)) is equal to orless than 4.5×10⁻⁷ H/m, and it is more preferable that that theirreversible susceptibility (χ_(irr)) is equal to or less than 4.0×10⁻⁷H/m.

It is preferred that the intrinsic coercive force (H_(CJ)) of the bondedmagnet at room temperature is 406 to 717 kA/m, and 435 to 677 kA/m ismore preferable.

If the intrinsic coercive force (H_(CJ)) exceeds the above upper limitvalue, magnetizability is deteriorated. On the other hand, if theintrinsic coercive force (H_(CJ)) is lower than the lower limit value,demagnetization occurs conspicuously when a reverse magnetic field isapplied depending upon the usage of the motor and the heat resistanceproperty at a high temperature is deteriorated. Therefore, by settingthe intrinsic coercive force (H_(CJ)) to the above range, in the casewhere the bonded magnet (cylindrical magnet in particular) is subjectedto multipolar magnetization, a satisfactory magnetization can beaccomplished even when a sufficiently high magnetizing field cannot besecured, which makes it possible to obtain a sufficient magnetic fluxdensity, and to provide a high performance bonded magnet, especially abonded magnet for motor.

The maximum magnetic energy product (BH)_(max) of the bonded magnet ofthe present invention is not particularly limited to the specific value.However, in the present invention, 87 to 125 kJ/m³ is preferable and 100to 125 kJ/m³ is more preferable.

EXAMPLES Example 1

Magnetic powders with alloy compositionsNd_(8.7)Fe_(77.2-w)Co_(8.5)B_(5.6)Si_(w) (that is, various types ofmagnetic powders in which the content w of Si is changed variously) wereobtained by the method described below.

First, each of the materials Nd, Fe, Co, B, and Si was weighed to cast amother alloy ingot, and a sample of about 15 g was cut out from theingot.

A quenched ribbon manufacturing apparatus 1 as shown in FIG. 4 and FIG.5 was prepared, and the sample was placed in a quartz tube 2 having anozzle (circular orifice) 3 at the bottom. After evacuating the interiorof a chamber in which the quenched ribbon manufacturing apparatus 1 ishoused, an inert gas (Ar gas and helium gas) was introduced to obtain anatmosphere with desired temperature and pressure.

Then, the ingot sample in the quartz tube 2 was melted by high frequencyinduction heating, the circumferential velocity and the jetting pressure(difference between the inner pressure of the quartz tube 2 and thepressure of the atmosphere) were adjusted to 20 m/s and 40 kPa,respectively. Under the state, the molten metal was jetted against thecircumferential surface 53 of the cooling roll 5, to obtain a quenchedribbon (average thickness of about 30 μm, and average width of about 1.6mm).

The quenched ribbon was then coarsely crushed, and the powder wassubjected to a heat treatment in an argon atmosphere at 680° C. for 300sec. In this way, the various types of magnetic powders each havingdifferent contents w of Si were obtained.

To analyze the phase structure of the obtained magnetic powders, therespective magnetic powder was subjected to X-ray diffraction usingCu-Kα line at the diffraction angle of 20°-60°. From the thus obtaineddiffraction pattern, the presence of diffracted peaks of a hard magneticphase, Nd₂(Fe.Co)₁₄B₁ phase, and a soft magnetic phase, α-(Fe.Co) phase,were confirmed. Further, from the observation result using atransmission electron microscope (TEM), the formation of a nanocompositestructure was confirmed in each magnetic powder.

Next, in order to adjust the grain size of the respective magneticpowders, each magnetic powder was ground by a granulator in an argon gasatmosphere to obtain magnetic powder having average grain size of 60 μm.

A composite (compound) for bonded magnet was prepared by mixing therespective magnetic powder with an epoxy resin and a small amount ofhydrazine antioxidant and then kneading them.

Then, each of the thus obtained compounds was crushed to be granular.Then, the granular substance was weighed and filled into a die of apress machine, and a molded body was obtained by compression molding (inthe absence of a magnetic field) the sample at a pressure of 7 ton/cm³.

After releasing from the die, the epoxy resin was cured by heating at atemperature of 150° C. (that is subjected to cure treatment) and acolumnar isotropic bonded magnet with diameter of 10 mm φ and height of7 mm was obtained. The content of the magnetic powder in the respectivebonded magnetic was 97.0 wt % and the magnetic density of the respectivebonded magnets was about 6.21 g/cm³.

After pulse magnetization is performed for the respective bonded magnetsunder the magnetic field strength of 3.2 MA/m, magnetic characteristics(remanent magnetic flux density Br, intrinsic coercive force (H_(CJ)),and maximum magnetic energy product (BH)_(max)) were measured using a DCrecording fluxmeter under the maximum applied field of 2.0 MA/m. Thetemperature at the measurement was 23° C. (that is, room temperature).

As shown in FIG. 7, in the measured demagnetization curve of J-Hdiagram, a recoil curve having a starting point P at an intersectioningpoint P between the demagnetization curve and a straight line whichpasses an origin and has a gradient of −3.8×10⁻⁶ H/m was produced withthe magnetic field being once changed to zero and then being returnedthe original state, and then the gradient of the recoil curve (that is,the gradient of the straight line connecting the both ends of the recoilcurve) was obtained and then it was defined as the reversiblesusceptibility (χ_(rev)). Further, the gradient of a tangential line ofthe demagnetization curve at the intersectioning point P was obtainedand then it was defined as the differential susceptibility (χ_(dif)).The irreversible susceptibility (χ_(irr)) was obtained by the formula ofχ_(irr)=χ_(dif)−χ_(rev). The results of them are shown in the attachedTable 1.

Next, the heat resisting property (heat stability) of the respectivebonded magnets (each having the column shape having diameter of 10 mmand height of 7 mm) is examined. The heat resisting property wasobtained by measuring the irreversible flux loss (ratio of flux loss)obtained when the bonded magnet was being left in the atmosphere of 100°C. for one hour and then the temperature was lowered to the roomtemperature, and then it was evaluated. The results thereof are shown inthe attached Table 1. In this connection, it is to be noted that smallerabsolute value of the irreversible flux loss (ratio of initial fluxloss) means more excellent heat resisting property (heat stability).

Next, in order to evaluate the magnetizability of the respective bondedmagnets (each having the column shape having diameter of 10 mm andheight of 7 mm), the magnetizability thereof was measured by changingmagnetic field for magnetization variously. In this connection, theratio of magnetizability was represented using the ratio with respect tothe remanent magnetic flux under the magnetic filed of 4.8 MA/m, wherethe remanent magnetic flux is represented by 100%. The size of themagnetic field of each of the bonded magnets when the ratio ofmagnetizability is 90% was shown in the attached Table 1. In this Table,smaller value means more excellent magnetizability.

Next, evaluation was made with regard to the corrosion resistanceproperty of the respective magnetic powders and the bonded magnets(having diameter of 10 mm and height of 7 mm) formed from the magneticpowders. The results thereof are shown in the attached Table 1 (FIG. 8).

1. Corrosion Resistance of Respective Magnetic Powders

The corrosion resistance property of the respective magnetic powderswere evaluated through a dewing test (dew formation test). This dewingtest was carried out by alternately placing each of the magnetic powdersin the atmospheres at a temperature of 30° C. and a RH of 50% for 15minutes and at a temperature of 80° C. and a RH of 95% RH for fifteenminutes, and this was repeated for 24 times. Thereafter, the surface ofthe respective magnets was observed by a microphone and the degree ofthe generation of the rust was evaluated from the viewpoint of thefollowing four steps.

A: no rust was generated

B: a very small amount of rust was generated

C: rust was generated

D: rust was remarkably generated

2. Corrosion Resistance of Respective Bonded Magnets

The bonded magnets (ten bonded magnets for each) were immersed into abath containing water at a temperature of 60° C. and a RH of 95%, andthen the average time required until any rust is generated in each ofthe bonded magnets was measured. The results thereof were evaluated fromthe viewpoints of the following four steps.

A: no rust was generated after 500 hours was past

B: rust was generated between 400 hours and 500 hours

C: rust was generated between 300 hours and 400 hours

D: rust was generated within 300 hours

As seen from the attached Table 1, each of the isotropic bonded magnetscomposed of the magnetic powders containing 0.2-3.0 at % of Si andhaving the irreversible susceptibility equal to or smaller than 5.0×10⁻⁷H/m exhibits excellent magnetic characteristics (remanent magnetic fluxdensity, intrinsic coercive force and maximum magnetic energy product).Further, they have high heat resisting property (heat stability) due tothe small absolute value of the irreversible susceptibility, and themagnetizability of the respective magnets is good. Further, since boththe magnetic powders and the bonded magnets have high corrosionresistance property, it is possible to omit or simplify corrosiontreatment such as application of anti-corrosion coating on the surfacesof the bonded magnets when they are actually used.

In contrast, the isotropic bonded magnets of the comparative exampleswhich were formed from the magnetic powders containing no Si orcontaining 3.5 at % of Si (more than the upper limit of the presentinvention) have poor magnetic characteristics. In particular, theisotropic bonded magnets formed from the magnetic powders containing noSi, the absolute value of the irreversible susceptibility is large andtherefore the heat stability thereof is low. Further, the magneticpowders containing no Si have poor corrosion resistance property inthemselves and when formed into the bonded magnets.

As described above, according to the present invention, it is possibleto provide bonded magnets having high performance and high reliability(especially, high heat resistance property and corrosion resistanceproperty). In particular, when these bonded magnets are used in motors,high performance can be exhibited.

Example 2

Quenched ribbons of which alloy compositions were(Nd_(1-y)Pr_(y))_(8.7)Fe_(bal)Co_(7.5)B_(5.6)Si_(1.4) (that is, varioustypes of quenched ribbons in which the substitution amount y of Pr ischanged variously) were manufactured in the same manner as Example 1,and then the manufactured quenched ribbons were subjected to heattreatment in the argon gas atmosphere at a temperature of 680° C. for 10minutes. Using the same analyzing method as that used in Example 1, ithas been confirmed that the structure of each of the quenched ribbonsconstitutes a nanocomposite structure.

Next, in the same manner as Example 1, magnetic powders were obtainedfrom the respective quenched ribbons, and then cylindrical (ring-shaped)isotropic bonded magnetic having outer diameter of 20 mm, inner diameterof 18 mm and height of 7 mm were manufactured. The content of themagnetic powder in each of the bonded magnets was about 96.8 wt %.Further, the density of each of the bonded magnets was about 6.18 g/cm³.

For these magnetic powders, magnetic characteristics (remanent magneticflux density Br, intrinsic coercive force (H_(CJ)), and maximum energyproduct (BH)_(max)) and the irreversible susceptibility (χ_(irr)) weremeasured, and then they were evaluated. The results thereof are shown inthe attached Table 2 (FIG. 9).

Further, these bonded magnets were respectively subjected to multi-polemagnetization of 12 poles, and using each bonded magnet as a magnet fora rotor, a DC brush-less motor was assembled. Then, each of the DCmotors was rotated at 4000 rpm to measure a back electromotive forcegenerated in the coil winding thereof. As a result, it has beenconfirmed that a sufficiently high voltage could be obtained in each ofthe motors and these motors have a high performance.

Next, bonded magnets the same as those of Example 1 were manufacturedexcepting that the various types of the magnetic powder having thedifferent substitution ratio y of Pr described above were used.

The heat resisting property (heat stability), the magnetizability andthe corrosion resistance property of each of the respective magneticpowders and the corrosion resistance property of the respectivemanufactured bonded magnets were measured in the same way as Example 1,and the measurement results thereof were evaluated. The results thereofare shown in the attached Table 2.

As seen from the attached Table 2, each of the isotropic bonded magnetshas excellent magnetic characteristics (remanent magnetic flux densityBr, intrinsic coercive force (H_(CJ)), and maximum magnetic energyproduct (BH)_(max)). In particular, it has been understood thatintrinsic coercive force (H_(CJ)) was further improved by replacing apart of Nd with Pr. Further, each of the bonded magnets has a smallabsolute value of the irreversible

Further, these bonded magnets were respectively subjected to multi-polemagnetization of 12 poles, and using each bonded magnet as a magnet fora rotor, a DC brush-less motor was assembled. Then, each of the DCmotors was rotated at 4000 rpm to measure a back electromotive forcegenerated in the coil winding thereof. As a result, it has beenconfirmed that a sufficiently high voltage could be obtained in each ofthe motors and these motors have a high performance.

Next, bonded magnets the same as those of Example 1 were manufacturedexcepting that the various types of the magnetic powder having thedifferent substitution ratio 1-z of Dy described above were used.

The heat resisting property (heat stability), the magnetizability andthe corrosion resistance property of each of the respective magneticpowders and the corrosion resistance property of the respectivemanufactured bonded magnets were measured in the same way as Example 1,and the measurement results thereof were evaluated. The results thereofare shown in the attached Table 3.

As seen from the attached Table 3, each of the isotropic bonded magnetshas excellent magnetic characteristics (remanent magnetic flux densityBr, intrinsic coercive force (H_(CJ)), and maximum magnetic energyproduct (BH)_(max)). In particular, it has been understood thatintrinsic coercive force (H_(CJ)) was improved by adding Dy. In thisconnection, it has been also confirmed that intrinsic coercive force(H_(CJ)) was appropriately improved when the replacing amount of Dy isequal to or less than 0.1 (that is, 10% with respect to the total of R).Furthermore, each of the bonded magnets has a small absolute value ofthe irreversible susceptibility, so that their heat resisting property(heat stability) was high and the magnetizability thereof was excellent.Moreover, both the corrosion resistance property of the magnetic powdersthemselves and the corrosion resistance property of the bonded magnetswere high. susceptibility, so that their heat resisting property (heatstability) was high and the magnetizability thereof was excellent.Furthermore, both the corrosion resistance property of the magneticpowders themselves and the corrosion resistance property of the bondedmagnets were high.

For these reasons, according to the present invention, it is possible toprovide bonded magnets having high performance and high reliability(especially, high heat resistance property and corrosion resistanceproperty). In particular, when these bonded magnets are used in motors,high performance can be exhibited.

Example 3

Quenched ribbons of which alloy compositions were((Nd_(0.5)Pr_(0.5))_(z)DY_(1-z))_(9.0)Fe_(ba1)Co_(7.7)B_(5.7)Si_(1.8)(that is, various types of quenched ribbons in which the substitutionamount (1-z) of Dy is changed variously) were manufactured in the samemanner as Example 1, and then the manufactured quenched ribbons weresubjected to heat treatment in the argon gas atmosphere at a temperatureof 680° C. for 12 minutes. Using the same analyzing method as that usedin Example 1, it has been confirmed that the structure of each of thequenched ribbons constitutes a nanocomposite structure.

Next, in the same manner as Example 1, magnetic powders were obtainedfrom the respective quenched ribbons, and then cylindrical (ring-shaped)isotropic bonded magnets having outer diameter of 20 mm, inner diameterof 18 mm and height of 7 mm were manufactured. The content of themagnetic powder in each of the bonded magnets was about 96.8 wt %.Further, the density of each of the bonded magnets was about 6.20 g/cm³.

For these magnetic powders, magnetic characteristics (remanent magneticflux density Br, intrinsic coercive force (H_(CJ)), and maximum magneticenergy product (BH)_(max)) and the irreversible susceptibility (χ_(irr))were measured, and then they were evaluated. The results thereof areshown in the attached Table 3 (FIG. 10).

For these reasons, according to the present invention, it is possible toprovide bonded magnets having high performance and high reliability(especially, high heat resistance property and corrosion resistanceproperty). In particular, when these bonded magnets are used in motors,high performance can be exhibited.

Example 4

Isotropic bonded magnets were manufactured in the same manner as thatused in Examples 1-3 excepting that the bonded magnets were producedusing the extrusion molding. In this regard, it is to be noted thatpolyamide (Nylon 610) was used in the respective bonded magnets as abinder. Further, the content of the magnetic powder in the respectivebonded magnets was about 95.5 wt % and the magnetic density thereof asabout 5.85 g/cm³.

For each of these bonded magnets, the above described measurement andevaluation were carried out. As a result, it has been confirmed that thesame results as those for the Examples 1 to 3 were obtained. Inparticular, corrosion resistance property thereof was particularlyexcellent.

Example 5

Isotropic bonded magnets of the present invention were manufactured inthe same manner as that used in Examples 1-3 excepting that the bondedmagnets were produced using the injection molding. In this regard, it isto be noted that poly phenylene sulfide was used in the respectivebonded magnets as a binder. Further, the content of the magnetic powderin the respective bonded magnets was about 94.1 wt % and the magneticdensity thereof was about 5.63 g/cm³.

For each of these bonded magnets, the above described measurement andevaluation were carried out. As a result, it has been confirmed that thesame results as those for the Examples 1 to 3 were obtained. Inparticular, corrosion resistance property thereof was particularlyexcellent.

As described above, according to the present invention, the followingeffects can be obtained.

Since each of the magnetic powders has the composite structure having asoft magnetic phase and a hard magnetic phase and contains apredetermined amount of Si, the magnetic powder exhibits excellentmagnetic characteristics so that intrinsic coercive force andrectangularity thereof are especially improved, and excellent corrosionresistance property is exhibited.

The absolute value of the irreversible flux loss is small and excellentheat resisting property (heat stability) can be obtained.

Because of the high magnetic flux density that can be secured by thisinvention, it is possible to obtain bonded magnet with high magneticperformance even if it is isotropic. In particular, since magneticperformance equivalent to or better than the conventional isotropicbonded magnet can be obtained with a magnet of smaller volume ascompared with the conventional isotropic bonded magnet, it is possibleto provide a high performance motor of a smaller size.

Moreover, since a higher magnetic flux density can be secured, inmanufacturing a bonded magnet sufficiently high magnetic performance isobtainable without pursuing any means for elevating the density of thebonded magnet. As a result, the dimensional accuracy, mechanicalstrength, corrosion resistance, heat resisting property (heat stability)and the like can be further in addition to the moldability, so that itis possible to readily manufacture a bonded magnet with highreliability.

Since the magnetizability of the magnet according to this invention isexcellent, it is possible to magnetize a magnet with a lower magnetizingfield. In particular, multipolar magnetization or the like can beaccomplished easily and surely, and further a high magnetic flux densitycan be obtained.

Since the bonded magnet of the present invention does not require tohave a high density, the present invention is adapted to themanufacturing method such as the extrusion molding method or theinjection molding method by which molding at high density is difficultas compared with the compression molding method, and the effectsdescribed in the above can also be obtained in the bonded magnetmanufactured by these molding methods. Accordingly, various moldingmethods can be selectively used and thereby the degree of selection ofshapes for the bonded magnet can be expanded.

Finally, it is to be understood that the present invention is notlimited to Examples described above, and many changes or additions maybe made without departing from the scope of the invention which isdetermined by the following claims.

What is claimed is:
 1. A magnetic powder comprising: an alloycomposition represented by R_(x)(Fe_(1-y)Co_(y))_(100-x-z-w)B_(z)Si_(w)(where R is at least one rare-earth element, x is 8.1-9.4 at %, y is0-0.30, z is 4.6-6.8 at %, and w is 0.2-3.0 at %), the magnetic powderbeing constituted from a composite structure having a soft magneticphase and a hard magnetic phase; and the soft magnetic phase and thehard magnetic phase have a mean crystal grain size of 1-100 nm; whereinthe magnetic powder has characteristics in which, when an isotropicbonded magnetic is molded by mixing the magnetic powder with a bindingresin, a irreversible susceptibility (X_(irr)) is equal to or less than5.0×10⁻⁷ H/m; the irreversible susceptibility is measured by using apoint where a demagnetization curve in a J-H diagram and a straight linethat passes through the origin in the J-H diagram intersect; thedemagnetization curve represents the magnetic characteristics at roomtemperature, and the straight line has a gradient (J/H) of −3.8×10⁻⁶;and the intrinsic coercive force (H_(cj)) of the magnet at roomtemperature is in the range of 406-717 kA/m.
 2. The magnetic powder asclaimed in claim 1, wherein the composite structure is a nanocompositestructure.
 3. The magnetic powder as claimed in claim 1, wherein said Rcomprises a rare-earth element mainly containing Nd and/or Pr.
 4. Themagnetic powder as claimed in claim 1, wherein said R includes Pr and aratio Pr with respect to the total mass of said R is 5-75%.
 5. Themagnetic powder as claimed in claim 1, wherein said R includes Dy and aratio of Dy with respect to the total mass of said R is equal to or lessthan 14%.
 6. The magnetic powder as claimed in claim 1, wherein themagnetic powder is obtained by quenching a molten state of the alloy. 7.The magnetic powder as claimed in claim 1, wherein the magnetic powderhas been obtained by forming a quenched ribbon of the alloy on a coolingroll and then pulverizing the quenched ribbon.
 8. The magnetic powder asclaimed in claim 1, wherein the magnetic powder has been subjected to aheat treatment for at least once during the manufacturing process orafter the magnetic powder's manufacture.
 9. The magnetic powder asclaimed in claim 1, wherein the average grain size of the magneticpowder lies in the range of 0.5-150 μm.
 10. An isotropic rare-earthbonded magnet, having been formed by binding the magnetic powder as setforth in claim 1, with a binding resin.